In turbines, a flow medium is transported in a flow duct in order to obtain energy therefrom. To this end, turbine blades are arranged in the flow duct. For example, in the flow duct of an axial gas turbine, guide-blade rings formed from guide blades and moving-blade rings formed from moving blades are arranged alternately following one another in the direction of flow. In a suitable manner, the guide blades deflect the flow medium onto the moving blades, which are connected to a rotor and are set in rotation, so that kinetic energy of the flow medium is converted into rotational energy.
Such blades in fluid-flow machines are often subjected to considerable mechanical loads. Especially at a simultaneous high temperature and high rotary speeds, as in a gas turbine, the blade material is subjected to high stress. As a result, cracks may form in the blade material, and these cracks propagate in the course of time when stress is continuous. Finally, failure of the blade may occur, the blade breaking into pieces or fragments being released. For blades following in the direction of flow, this may lead to considerable damage. The formation and propagation of cracks thus need to be monitored. Depending on the speed of the processes, a significant reduction in the availability of the turbine may occur as a result, since regular service intervals lead to turbine downtimes.
Described in U.S. Pat. No. 6,490,791 is a method in which cracks in the trailing edge of a turbine blade are removed in a service process by cutting back the trailing edge. The additional aerodynamic losses caused by the shortened trailing edge are kept low by subsequent rounding of the blade profile. Although this method can avoid a complete exchange of used blades for new blades, it does not reduce the frequency of the service intervals.
Shown in JP 2000018001 is a gas-turbine moving blade in which relief slots are incorporated in the direction of the blade axis toward the margin of the tip region. These relief slots serve to reduce thermal stresses in this region. The reduction in thermal stresses is intended to reduce crack formation. The relief slots are restricted to the tip region.
JP 10299408 shows a gas-turbine blade in which elliptical holes which are intended to reduce crack propagation are incorporated in regions of high thermal stresses. The holes are arranged in the transition region between blade airfoil and platform, the major axis of the ellipse being directed perpendicularly to the blade axis in the region of the blade airfoil. There is a corresponding orientation of the holes at the trailing edge.